The present invention relates generally to techniques for determining loss of consciousness during anesthetization and specifically to an objective measure of the effectiveness of anesthetization based on analysis of electroencephalographic signals.
Anesthetics are critical to modern surgical practice. Unfortunately, once in every 1000-2000 operations a patient may temporarily regain consciousness or even remain conscious during surgery. Such intraoperative awareness arises in part because of an inability to evaluate levels of consciousness.
Determining consciousness is difficult because of the different ways an anesthetic may work. For example, certain anesthetics may impair a person's willingness to respond even though they are conscious. Paralyzing agents, used to prevent unwanted movement during anesthesia, can also affect the patient's ability to respond to a command while the patient is conscious. It is possible that anesthetics which promote forgetfulness can cause a patient to fail to respond because they forget what to do before they can respond. Finally, some anesthetics promote profound amnesia, raising the possibility that retrospective oblivion is misinterpreted as loss of consciousness.
Anesthetics are thought to work by interacting with ion channels that regulate synaptic transmission and membrane potential in key regions of the brain and spinal cord and are known to affect electroencephalographic signals collected at the scalp. Prompted by this knowledge, brain function monitors have been developed to monitor EEG signals to determine levels of consciousness. Examples include the bispectral index (BIS) monitor manufactured by Aspect Medical Systems, Inc. of Newton, Mass. and the patient state index (PSI) monitor manufactured by Physiometrix of Billerica, Mass. both which employ proprietary algorithms to analyze EEG waveforms in order to output an index value indicating general levels of anesthesia. Although such devices may help guide anesthetic delivery, it is not clear whether they can indicate the presence or absence of consciousness, particularly near the transition point between these two states. Their limitations follow at least in part from an imperfect understanding of what consciousness is.